A sufficient angle of view is required for an objective optical system applied to an endoscope apparatus. As such wide-angle objective optical systems applicable to endoscopes, for example, in PTLs 1 to 4, disclosed are endoscope objective optical systems in each of which is provided: a negative first lens; a negative second lens; a positive third lens; a brightness diaphragm; and a positive lens group from an object side, and in each of which an angle of view is set to be not less than 200 degrees while suppressing a lens diameter. In each of these endoscope objective optical systems, the first lens closest to the object side has a convex meniscus shape, and thus distortion is successfully corrected although the lens has a wide angle.